1. Field of the Invention
The present invention relates to an image forming apparatus forming an image on a recording material, for example an image forming apparatus such as a copier, printer or fax machine.
2. Related Background Art
Conventionally, in an electrophotographic image forming apparatus, a toner image as a developer image is formed on a recording material such as a sheet by an image formation process called Carlson process, and then the toner image is fixed as a permanent image. For fixing systems for this purpose, various systems have been proposed, but hot-pressing systems heating and pressing a toner image to be fixed are common in terms of fixing characteristics, and in particular, roller type fixing apparatuses (roller fixing) making a toner image directly contact a pair of rotating members including a heating source to fix the toner image are often used.
That is, it has as a basic structure a roller pair of a fixing roller (heat roller) thermally adjusted to have a predetermined fixing temperature and a pressure roller abutted against the fixing roller, wherein the roller pair is rotated, a recording material having an unfixed toner image formed thereon is introduced into a fixing nip portion being an abutment portion of the roller pair, and nipped and conveyed to hot pressing-fix the unfixed toner image on the recording material by means of the heat of the fixing roller and the pressure of the fixing nip portion.
The amount of heat given to the recording material bearing the unfixed toner image depends on temperatures of the fixing roller and the pressure roller, and a time period over which the recording material passes through the fixing nip portion, i.e. a fixing nip width and a travel speed of the recording material. The fixing nip width refers to a length of the fixing nip portion along which the recording material travels.
For a fixing apparatus to be mounted on an electrophotographic apparatus having a higher process speed, a fixing apparatus having a larger fixing nip width is required in relation to the amount of heat described above. For increasing the fixing nip width in the roller fixing described above, the diameter of the roller should be increased, and if the diameter of the roller is increased, there arises a problem such that the heat capacity of the roller increases to lengthen the heat-up time (warm-up time) of the roller.
Thus, as a fixing apparatus configuration capable of securing a large fixing nip width without increasing the diameter of the roller, a belt type fixing apparatus is proposed in Japanese Patent Application Laid-Open No. S61-132972.
In such a belt type fixing apparatus, a heat-resistant and flexible endless belt stretched between a plurality of belt stretching members is abutted against a fixing roller being a rotating member for heating to form a fixing nip portion, and the fixing nip portion is made to nip and convey a recording material bearing an unfixed toner image, whereby the unfixed toner image is hot press-fixed on the recording material by means of the heat of the fixing roller and the pressure of the fixing nip portion. This belt type fixing apparatus allows the fixing nip width to be easily set to a larger width by adjusting a width of the endless belt facing the fixing roller. The fixing nip width can be secured independently of the diameter of the fixing roller, and therefore the diameter and the heat capacity of the fixing roller can be reduced, thus making it possible to shorten the start-up time.
Recording materials for use in the image forming apparatus include resin coated sheets (hereinafter referred to as coated sheets) having a high glossiness on the front surface of both front and rear surfaces with an acryl resin, polyolefin resin or the like coated on the surface of the front surface or both front and rear surfaces of the recording material.
However, if the fixing apparatus is made to pass with a toner placed on the coated sheet, an image defect called a blister in which the recording material is partially deformed like a blister may occur. This is caused due to the fact that water in the interior of the coated sheet is evaporated by heating by the fixing apparatus to increase the volume, but due to a coat layer on the surface of the coated sheet, vaporized water vapor is not uniformly dissipated to outside the coated sheet, but collectively discharged to outside the coated sheet from a portion in which the coat layer is thinned or absent, and therefore the coat layer is ruptured.
In such a fixing apparatus, the fixing nip width is large from a configuration viewpoint, a time period over which the fixing roller and the belt contact each other is therefore lengthened, and thus the temperature of the belt is easily elevated. Because the heat capacity of the belt is small compared to a conventional pressure roller, the temperature of the belt is elevated in a short time period. That is, there arises a problem such that the above image defect tends to occur.
For alleviating the problem, it is proposed in Japanese Patent Application Laid-Open No. H11-194647 that the belt and the fixing roller are separated during standby as a method for reducing the amount of heat given from the rear surface of the recording material.
However, the image forming apparatus described in Japanese Patent Application Laid-Open No. H11-194647 has the following problem.
In a continuous image formation job for continuously forming toner images on a plurality of recording materials, the fixing roller and the belt contact each other even during a time period over which no recording material exists in the fixing nip (so called sheet interval), and therefore heat of the fixing roller is transferred to the belt, whereby the temperature of the belt is elevated to cause an image defect as described above. In particular, if the configuration in which the fixing speed is decreased compared to the normal speed or the above sheet interval is lengthened in consideration of fixing characteristics is employed in the continuous image formation job for continuously forming toner images on a plurality of recording materials, the problem described above is notable.